Saturday, May 16, 2015

Atomic Layer Deposition of Al2O3 on NF3-pre-treated graphene

Another great publication from ALD Lab Dresden, TU Dresden, Germany, and Marcel Junige and their and scientists at Linköping University of Technology, Sweden, using high resolution in-situ ellipsometer. This time these guys have grown Al2O3 on Graphene, which is very difficult unless you activate the inert grapheme surface. Marcel did this by a NF3 pre-treatment. The work was presented at SPIE micro technologies 2015 in Barcelona.

Graphene has been considered for a variety of applications including novel nanoelectronics device concepts such as the recently reported Graphene Base Transistor (GBT). However, the deposition of ultra-thin films on top of graphene is still challenging: On the one hand, the deposition process must not damage or alter the pristine graphene monolayer; on the other hand, the finally deposited films have to provide appropriate functional properties regarding a specific application. In case of the GBT, a dielectric coating is desired which is both pin-hole free to prevent any short circuits and still thin enough (around 3-5 nm) to enable hot electron tunneling. Hence, the dielectric film closure on graphene needs to occur at an early stage of the deposition process. Atomic Layer Deposition (ALD) has been established as a physicochemical coating technique with excellent thickness control as well as unique conformality over complex three-dimensional-shaped substrates for the last decade. Especially the ALD of oxides has been extensively researched. Accordingly, an ALD process for Al2O3 yet exists that alternates the exposure of trimethylaluminum (TMA) and water (H2O) as the organometallic precursor and co-reactant of two corresponding self-terminating surface reactions, respectively. However, the ALD of Al2O3 has been reported to barely initiate on pristine graphene due to graphene’s lack of dangling bonds. A fluorine functionalization, using XeF2, has been found to provide additional nucleation sites resulting in conformal films without pinholes. Based on this literature finding, we studied the impact of pre-treatments by nitrogen trifluoride (NF3) on exfoliated as well as epitaxial graphene monolayers prior to the ALD of Al2O3. All experiments were conducted in vacuo; i. e. the pristine graphene samples were exposed to NF3 for 180 s in the same reactor immediately before applying 30 ALD cycles and the samples were transferred between the reactor and a surface analysis unit under high vacuum conditions. The ALD growth initiation was observed by in-situ real-time Spectroscopic Ellipsometry (irtSE) with a sampling rate of 1 Hz. The chemical surface composition before and after the ALD as well as the presence of graphene after the coating procedure were revealed by in-vacuo X-ray Photoelectron Spectroscopy (XPS). The morphology of the films was determined by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The defect status was examined by Raman Spectroscopy before and after the coating procedure. Atomic Layer Deposition of Al2O3 on NF3-pre-treated graphene. Available from: https://www.researchgate.net/publication/276242739_Atomic_Layer_Deposition_of_Al2O3_on_NF3-pre-treated_graphene [accessed May 16, 2015].